1. Field of the Invention
This invention relates to a cutting tool with a base coated with a layer.
2. Description of the Related Art
Cutting tools widely used for cutting metals are often made of a base of cemented carbide, cermet, ceramics or the like, with its surface coated with a single layer or a plurality of layers of titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), titanium aluminum nitride (TiAlN) or the like.
Surface roughness of such a cutting tool controls friction with a workpiece and cooling effect of a cutting fluid, and exerts an influence on the cutting. In some cases where the surface roughness is significantly high, a cutting edge of the cutting tool becomes so hot in continuous cutting that the workpiece adheres to the cutting edge to reduce wear resistance of the cutting tool significantly. In cutting such as interrupted cutting that exerts a high impact on the cutting edge, the effect of the impact varies microscopically depending on distribution of uneven spots on the surface of the cutting edge. As a result, cracks are caused in a region of the uneven spots where a great deal of stress converges. The cracks induce chipping which causes a problem of significant reduction in fracture resistance.
WO02-004156 discloses that the surface roughness (Rmax) of a tip of a cutting edge is reduced to equal to or less than 0.2 μm by mechanical processing such as polishing using a brush or lapping. It discloses that the adhesion and micro-chipping due to the surface roughness can be prevented by the processing to extend the tool life.
By reducing the surface roughness of the tip of the cutting edge, however, it is not avoidable that the cutting edge is heated to a high temperature in high speed cutting or in cutting a workpiece of low heat conductivity such as stainless steel. The cutting edge heated to the high temperature is reduced in the wear resistance and is prone to the adhesion of the workpiece. There has been a problem that achieving a long tool life of the cutting tool is not easy because the reduction in the wear resistance and the adhesion of the workpiece induce a sudden fracture and an abnormal wear of the cutting edge.
On the other hand, the high speed cutting has been brought into use, as a high degree of efficiency is required to the cutting in recent years. There is performed cutting in which a cutting fluid is used to cool the workpiece. Even with the cutting fluid, however, the cutting edge becomes very hot and becomes to prone to the reduction in the wear resistance and to causing the adhesion to the cutting edge. There has been a problem that the reduction in the wear resistance and the adhesion induce the fracture and the abnormal wear of the cutting edge, which result in a short tool life.
Retentivity of the cutting fluid is important in the high speed cutting. Examples are disclosed in Japanese Patent Application Publication Nos. H7-157862 and 2002-146515. That is, when a layer is formed on a surface of a base by an arc ion plating method, droplets (coarse particles) protruding from the surface of the layer are also formed. Mechanical processing such as barrel processing and honing processing is performed to remove top portions of the droplets protruding from the surface. By the processing, however, whole droplets including portions buried in the layer are removed together with the top portions in a way that the droplets are wholly pulled out from the layer. As a result, concave portions (fine holes) are formed on the surface of the layer. The fine holes serve as storage for the cutting fluid to increase the retentivity of the cutting fluid. As a result, lubrication in the cutting is enhanced to improve the wear resistance.
In the case where the size of the fine holes is large, however, cracks are apt to be caused from edges of the fine holes to reduce the fracture resistance.